Title

Solubility and diffusion of organic vapors in silicone polymers

Date of Award

1998

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical and Chemical Engineering

Advisor(s)

P. A. Rice

Second Advisor

S. Alexander Stern

Keywords

Silicone, Alkanes, Aromatics, Alcohols

Subject Categories

Chemical Engineering

Abstract

Study of solubility and diffusion of organic vapors in polymer membranes is important for the development of membrane processes for the removal of organic vapors from air for environmental applications. The solubility and absorption/desorption kinetics of seven organic vapors, namely, n-pentane, n-hexane, n-heptane, benzene, cyclohexane, methanol, and 1-propanol in two polymer membranes, i.e. poly(dimethyl siloxane), PDMS, and poly(trifluoropropyl methyl siloxane), PTFPMS, have been studied at 35.0$\sp\circ$C at different vapor activities.

Solubilities of seven vapors in both polymers increase exponentially with increasing vapor activity. S(0) of the nonpolar vapors in PDMS are considerably higher than the corresponding values of these vapors in PTFPMS because of the lower glass-transition temperature, $T\sb{g}$, and hence the larger mean free volume of the former polymer. S(0) of polar vapors (methanol and propanol) are of similar magnitude for both silicones polymers.

The Flory-Huggins equation describes satisfactorily the solubilities of nonpolar penetrant vapors but fails to describe the solubilities of polar vapors in both polymers. However, solubilities of all vapors in both polymers can be represented by either Flory-Rehner equation or modified Flory-Huggins equation. Values of the Flory interaction parameters for seven organic vapors in both polymers decrease with increasing solubility of these vapors.

Diffusion coefficients obtained from absorption measurements were corrected for heat and swelling effects caused by the very high solubilities of the penetrant vapors in both polymers. Diffusion coefficients determined from the desorption measurements were corrected only for swelling effects, because the heat effects were negligible under the experimental conditions of this study. Heat-effect corrections were made using theory of Armstrong et al. with pertinent initial and boundary conditions. Corrected mutual diffusion coefficients of nonpolar vapors increase exponentially with increasing vapor concentration. By contrast, diffusion coefficients of polar vapors decrease with increasing concentration because of the clustering.

Temperature changes of membrane sample (increase during absorption and decrease during desorption) obtained from measurements and those obtained from heat-effect models are in satisfactory agreement. The magnitude of the heat effects decreases with increasing membrane thickness.

The correlation of diffusion coefficients and concentration using Fujita's and Vrentas and Duda's free-volume models must be considered as semiempirical.

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